The present invention relates to signal processing, and, in particular, to automatic gain control circuits for amplifiers, such as RF amplifiers.
An important parameter associated with radio-frequency (RF) amplifiers is the amplification factor or gain. Numerous methods have been devised to provide automatic gain control (AGC) of RF amplifiers that function to maintain constant gain despite changes in operating parameters, such as temperature, voltage, signal level, and component age, to name a few.
Closed-loop AGC
FIG. 1 is a block diagram of a closed-loop AGC system 100 of the prior art. AGC system 100 has an RF signal generator 102, closed-loop AGC circuit 104, and load 106. The objective of AGC system 100 is to amplify the RF signal produced by signal generator 102 by a fixed amount and deliver the amplified signal to load 106 (e.g., a resistor).
In particular, the RF signal from generator 102 is input to AGC circuit 104 at input terminal 108. The RF input signal flows through directional coupler 110 and then to voltage-controlled attenuator (VCA) 112. The purpose of VCA 112 is to vary the level of the RF input signal. The signal is then routed from the output of VCA 112 to the input of RF amplifier 114. The output of RF amplifier 114 is routed through output coupler 116 and then to output terminal 118, which is connected to load 106.
The signal gain G between terminals 108 and 118 of AGC circuit 104 is determined by G=Axe2x88x92B, where A is the gain of amplifier 114 (e.g., in dB) and B is the loss of VCA 112 (e.g., in dB). As the value of gain A of RF amplifier 114 changes in response to various operating parameters, the value of loss B of VCA 112 is adjusted accordingly to maintain the overall gain between terminals 108 and 118 constant.
The control signal to automatically maintain VCA 112 at the proper level of insertion loss is provided by elements within closed-loop AGC circuit 104. In particular, the input power level of the RF signal sampled by directional coupler 110 is detected by input detector 120 before being routed to the positive input terminal 124 of differential amplifier 122. The amplified RF output signal is sampled by directional coupler 116, attenuated (by approximately xe2x88x92A dB) by passive attenuator 128 (e.g., three resistors) before being detected by output detector 130. The attenuated and detected version of the RF output signal sample is then routed to the negative input terminal 126 of differential amplifier 122. The output of differential amplifier 122 will be set to a specific DC voltage Vr depending upon the difference between the sampled input power level present on positive input terminal 124 and the sampled output power level present on negative input terminal 126. The output of differential amplifier 122 is routed to the control voltage input 132 of VCA 112 to control the level of attenuation (i.e., inverse gain) applied by VCA 112 to the RF input signal received from RF generator 102.
For the following description of AGC action, the power level of the RF input signal received from RF generator 102 is assumed to remain constant. Environmental changes, such as elevated temperature, will cause the gain of RF amplifier 114 to decrease, resulting in a decrease in the power level of the RF output signal at terminal 118. Accordingly, the attenuated and detected sample of the RF output signal presented to the negative input terminal of differential amplifier 122 will also decrease in value. The detected sample of the RF input signal presented to the positive input terminal of differential amplifier 122 will remain the same since the RF input signal is assumed to be held constant. As such, the output of differential amplifier 122 will increase in voltage, which increases the voltage on the control voltage input of VCA 112. The transfer characteristics of VCA 112 are designed such that an increase in voltage on the control voltage input results in a decrease in the loss value B. The resulting decrease of attenuation of VCA 112 causes the input power level presented to the input of RF amplifier 114 to increase, which in turn causes the RF power level presented to the RF output signal to increase at output terminal 118. The RF output power of amplifier 114 will continue to increase until the sampled, attenuated, and detected version of the RF output signal presented to negative input terminal 126 of differential amplifier 122 equals the sampled and detected version of the RF input signal presented to positive input terminal 124 of differential amplifier 122. The output voltage of differential amplifier 122 will then be set to a value lower than the original value Vr, which restores the original gain between terminals 108 and 118 of AGC circuit 104.
AGC operation is similar in response to environmental changes or other factors that cause the gain of RF amplifier 114 to increase, such as low-temperature operation. In this case, the sampled, attenuated, and detected version of the RF output signal increases in value even though the RF input signal power level remains constant. The rising value of the sampled, attenuated, and detected version of the RF output signal presented to negative input terminal 126 of differential amplifier 122 causes the output of differential amplifier 122 to decrease in voltage. This decrease in voltage on VCA control voltage input 132 causes VCA 112 to increase its level of attenuation B. This increase in attenuation causes the RF signal power level presented to the input of RF amplifier 114 to decrease, which in turn causes the RF output signal level present at output terminal 118 to decrease in power. The trend continues until the sampled, attenuated, and detected version of the output signal presented to negative input terminal 126 of differential amplifier 122 equals the sampled and detected version of the input signal presented to positive input terminal 124 of differential amplifier 122. The output voltage of differential amplifier 122 will now be set to a value higher than the original value Vr, which restores the original gain between terminals 108 and 118 of AGC circuit 104.
Closed-loop AGC circuit 104 can also be used to maintain the gain between terminals 108 and 118 due to change in the gain of RF amplifier 114 resulting from input signal level changes. Large-signal amplifiers implemented with bipolar devices and operating as Class AB devices for improved efficiency typically increase in gain as the input signal level is increased. This gain expansion causes the RF output signal to further increase beyond the expected amplifier amplification factor of Axe2x88x92B. Further increasing the input signal level (beyond the so-called gain compression point) eventually causes the RF amplifier gain to decrease or compress below the expected amplifier factor A as is well known to those skilled in the art.
Large-signal RF amplifiers implemented with new technology devices, such as laterally diffused metal oxide silicon (LDMOS) transistors, exhibit significantly improved linearity over the same dynamic range of input signal. As such, the gain of RF amplifier 114 remains substantially constant regardless of input signal level up until the gain compression point. The issue of gain compression is not of concern for many modern large-signal RF amplifier applications involving digital modulation. In such cases, RF amplifier 114 is sized such that the maximum RF signal output is well below the 1-dB gain compression point. Hence, closed-loop AGC operation as depicted in FIG. 1 is not necessary to control gain expansion or gain compression of such LDMOS RF amplifiers in many digital modulation applications such as TDMA, CDMA, UMTS, or other well-known digital modulation formats.
On the other hand, employing closed-loop AGC on RF amplifiers in digital modulation applications presents special challenges, such as stability of the control loop over widely varying RF input signals or in the absence of RF input signals. If the loop operates too fast, then AGC operation may function to alter the amplitude characteristics of the digitally modulated signal causing distortion of the signal intelligence. If the loop operates too slow, then gain accuracy might not be met over the dynamic range of the input signal. Gain accuracy also depends on other factors such as how well input detector 120 matches output detector 130 and how they track over temperature.
Open-loop AGC
Modern amplifiers utilizing LDMOS technology in digital modulation applications still typically utilize some means of automatic gain control to maintain gain over temperature. FIG. 2 shows a block diagram of open-loop AGC system 200 of the prior art. Like closed-loop AGC system 100 of FIG. 1, open-loop AGC system 200 has an AGC circuit 204 connected between an RF signal generator 202 and a load 206 to maintain constant amplification of the RF input signal received from generator 202 at input terminal 208 for application to load 206 at output terminal 218. Moreover, like AGC circuit 104, AGC circuit 204 has a voltage-controlled attenuator 212 and an RF amplifier 214. Unlike AGC circuit 104, however, AGC circuit 204 has a temperature sensor 234 adapted to sense the temperature of amplifier 214. The sensed temperature signals are presented to microprocessor 236, which can access data stored in look-up table (LUT) 238 and generate an output signal that is converted into a voltage control signal by digital-to-analog converter (DAC) 240 for application to control voltage input 232 of VCA 212.
In operation, microprocessor 236 monitors the operating temperature of RF amplifier 214 via temperature sensor 234. As temperature changes, microprocessor 236 reads correction data from LUT 238 and uses the correction data to alter the voltage output of DAC 240. The output voltage of DAC 240 serves as input to VCA control voltage input 232, which in turn controls the RF signal level input to RF amplifier 214. Microprocessor 236 can thus control DAC 240 to increase the VCA control voltage as temperature increases to minimize the gain change between terminals 208 and 218. Likewise, microprocessor 236 can control DAC 240 to decrease the VCA control voltage as temperature decreases to minimize the gain change between terminals 208 and 218.
This open-loop method of automatic gain control requires detailed characterization of several amplifiers over temperature to determine the average behavior to determine the correction voltage necessary to be applied to VCA control voltage input 232 to maintain constant gain. This data is used as the basis for LUT 238.
The open-loop method of FIG. 2 can be simpler and less expensive to implement than the closed-loop method of FIG. 1 considering that many modern, large-signal amplifiers already employ microprocessors, look-up table memory, and digital-to-analog converters in support of other amplifier functions. The principal disadvantages associated with this open-loop method are the lack of unit-specific accuracy. Gain correction is done based on an average of similar amplifiers that takes a great deal of time and effort to obtain. Moreover, correction is done open loop without actually knowing what the RF output signal power level is. These factors limit the gain accuracy that can be maintained over temperature.